The objective is to find out why preimplantation mouse embryos at the 8-cell and morula stage will develop in vitro into blastocysts in media containing any one of a variety of energy sources, including glucose, pyruvate, lactate, malate, citrate and a number of amino acids, whereas at the 1-cell and 2-cell stages only pyruvate or lactate will serve this purpose. The basic approach is to measure the levels of key metabolites in single embryos which have been stained and then refed with the nutrient in question. The analytical methods are based on specific enzymatic reactions involving DPN or TPN. The necessary sensitivity (to measure as little as 10 minus to the 15th power moles of material) is provided by "enzymatic cycling".